Weathering, Soils and Mass Wasting - WOU

Weathering, Soils and Mass Wasting

I.

Basic Definitions/Introduction

A.

Continental Dynamics / Landscape Evolution

1.

Mass Transfer Functions

a.

Dynamic interaction between uplift of earth's crust, or emergence on crust,

and erosion of materials from higher elevations and moving them to lower

elevations

b.

mass transfer = energy driven

B.

Weathering - disintegration and decomposition of rock at or near the surface of the earth,

fragmenting rock into particles via physical and chemical processes

1.

In-situ ("in place, without transport") disintegration of rock or unconsolidated

materials

2.

Earth Surface Conditions

a.

Water-rich Atmosphere and Hydrosphere

b.

Low Temperature

3.

Water - universal solvent, very effective at chemically altering rocks

C.

Sediment - fragments of rocks and/or minerals that are produced from the weathering of

pre-existing rock

1.

D.

Erosion - incorporation and transportation of sediment by a mobile agent, driven by

mechanical energy

1.

E.

Grain Size Classification According to Clast Diameter (from large fragments to

small)

a.

Gravel: pebble to boulder size range (>2 mm)

(1)

boulders: > 256 mm (25.6 cm)

(2)

cobbles: 16 mm - 256 mm

(3)

pebbles: 4 mm - 16 mm

(4)

granules: 2 mm - 4 mm

b.

Sand: 0.0625 mm to 2 mm in diameter

c.

Silt: 4 micrometers (0.004 mm) to 0.0625 mm)

d.

Clay: < 4 micrometers in diameter

Surface Agents of Transportation and Erosion

a.

Wind (eolian)

b.

Water (river-fluvial)

c.

Ice (glacial)

d.

Gravity (mass wasting = transfer of surficial material downslope under the

influence of gravity)

(1)

also drives flow of water and ice

Denudation of earth's surface: physical systems driving towards equilibrium: in this

case the system includes elevating the earth's surface to provide potential energy, with

subsequent driving towards peneplanation via gravity, water, and weathering.

19

F.

Surficial Materials at Earth's Surface

1.

Bedrock - in situ, consolidated, indurated, rock (igneous, sedimentary,

metamorphic)

2.

Regolith - all weathered and unconsolidated surficial materials at the Earth's

surface

a.

residuum - in-situ regolith, weathered rock material that has not been

significantly transported

b.

colluvium - regolith deposits that have been transported primarily by gravity

and mass wasting processes

c.

alluvium - regolith deposits that have been transported primarily by fluvial

processes

d.

Other Surficial Materials

(1)

Eolian deposits - wind-blown sediment

(a)

loess - windblown silt

(b)

sand dunes

(2)

Glacial deposits - ice-transported sediment

(a)

"drift" - all glacially transported sediment

(b)

"till" - ice-transported sediment, deposited as ice melts

(c)

"outwash" - ice-transported sediment, reworked by fluvial

processes from streams of glacial meltwater

(3)

Lacustrine Deposits - lake sediment

(a)

lacustrine delta sediments (sediments deposited at the

mouths of rivers emptying into lakes)

(b)

beach sediments - wave reworked deposits

(c)

offshore sediments - quiet water deposition of fine sediments

and/or chemical deposits (e.g. playas)

(4)

Coastal Deposits

(a)

tidal marsh deposits - fine mud and sand, influenced by tidal

and estuary processes

(b)

beach sand / gravel - wave reworked sediment at the beach

(c)

marine delta deposits - sediments deposited at the mouths of

rivers emptying into the ocean

G.

Soil Characteristics

1.

Pedogenesis - chemical and physical processes that alterate regolith into soil

2.

unconsolidated surficial material with well-defined mineral and chemical horizons

a.

O-horizon: organic layer, forms the interface between plants and regolith

b.

A-horizon: zone of leaching, downward percolating soil water

c.

B-horizon: zone of accumulation, deposition of clays and mineral deposits

d.

C-horizon: unaltered regoltih

e.

R-horizon: fresh, unweathered bedrock

3.

Comprised of a mixture of air, water, mineral, and organic matter

a.

porosity- open space between mineral grains

(1)

storage points for water and air in soil

b.

interface for plant growth

(1)

root systems

(2)

organic matter

(3)

micro-organisms

20

H.

II.

Clay - a special note

1.

"Clay" is used in two ways:

a.

all sediment < 4 microns in diameter (as discussed above)

b.

a chemically distinctive mineral species

2.

Basic Rock Composition

a.

Rocks made up of minerals, minerals composed of elemental compounds

b.

Average Rock Forming Minerals

(1)

typically form in igneous environment at high temperatures

(2)

silicate-based compounds (with no water in their structure)

(a)

Quartz

SiO2

(b)

Orthoclase Feldspar

KAlSi3O8

(c)

Plagioclase Feldspar

(Ca-Na)AlSi3O8

(d)

Pyroxene

Ca(Mg,Fe)AlSi2O6

(e)

Hornblende

(Ca,Na)(Mg,Fe)Al3Si6O22

3.

Clay Minerals

a.

Form near Earth surface in low temperature, low pressure environment

b.

incorporate water (H2O) into the crystal structure

c.

represent the stable end-product of rock weathering at the Earth's surface

d.

examples

(1)

Kaolinite

Al2Si2O5(OH)4

(2)

Illite

KAl3Si3O10(OH)2

Weathering - physical and chemical processes that breakdown rock to form regolith.

A.

Physical Avenues of Rock Weathering - enhanced pathways for water percolation.

1.

Rock Fractures serve as the avenues from which the weathering process can take

place.

a.

Microfractures- down to the mineral grain level

(1)

Atomic bond weaknesses serve as avenues for chemical alteration

by water

b.

Joints- open rock fractures/cracks- form as a result of stress release on

bedrock as overburden is removed or due to subtle tectonic stress.

c.

Faults- zones of weakness which may shatter wide zones of rock

d.

Pore spaces- e.g. vesicules, intergranular, secondary

(1)

Pore openings provide the permeability conduit for water to flow

through rocks and surficial material

e. Solution Cavities-dissolution of rock by water, e.g. limestone caves

2.

Climate and Weathering

a.

The dominant style of weathering process will be controlled by climate in

form of available moisture, temperature and vegetative growth

21

B.

Physical or Mechanical Weathering

1.

Defined - Physical fragmentation of bedrock as it is exposed at or near the earth's

surface

a.

Physical weathering accomplished by differential stress induced within rock

= results in cracking

2.

Frost Wedging - process of alternate freezing and thawing of water/moisture

contained in cracks and fractures of rock.

a.

Water expands 9% in volume as it freezes, exerting outward stress on rock

surfaces, "wedging" apart pieces of rock to form sediment.

b.

Ice expansion in confined space can create pressures of up to 30,000 lb/sq.

in.

c.

water infiltrates rock through meteoric precipitation and infiltration through

joints, cracks and microfractures

d.

Necessary Climatic Factors/Temp. Fluctuation

(1)

supply of water

(2)

alternating freeze/thaw cycles

(3)

temps. below 0 C to ensure freezing at depth

(a)

common in high latitude/high altitude areas

e.

Landscape Evidence

(1)

Talus Slopes - rubble accumulations at the base of rocky slopes /

cliffs

(a)

Talus = landform of rubbly slope

(b)

Scree = rubble material that forms talus

3.

Unloading or release of overburden pressure during denudation

a.

Erosion / denudation of Earth crust

(1)

isostatic response to unloading = crustal uplift

(2)

crustal unroofing = pressure release and rock expansion

(a)

Original depths of eroded mountain cores range from 8-30 km

(b)

equivalent to 8 kbar pressure, capable of producing 0.8%

expansion upon release

(3)

rock expansion = brittle cracking

b.

particularly common in weathering of granite bodies, e.g. Yosemite National

Park.

Sheeting - rock breaking into concentric, onion-like slabs. The slabs peel

off to produce domelike structures known as Exfoliation Domes.

(1)

exfoliation fractures or sheet joints

(a)

joints found predominantly within first few meters of exposure

c.

22

4.

Thermal Heating and Rock Expansion

a.

b.

c.

basic premise:

(1)

physical heating of materials = volume expansion

(2)

physical cooling of materials = volume contractions

rocks poor conductors of heat:

(1)

outer layers of rock heat and expand, while inner layers remain

cooler: differential expansion results in cracking.

Heat Sources / Geographic Areas

(1)

Forest Fire / Fire-Prone Areas

(2)

Hot desert regions (e.g. Death Valley) - solar heating

5.

Organic/Biologic Activity - activities of plants, animals, and humans can act as a

weathering agent.

a.

The physical growth processes result in placing stress on the rocks causing

them to fracture.

b.

Plants/trees readily root and grow in cracks and fractures of rock (e.g.

weeds growing up through concrete).

c.

Burrowing organisms constantly stir/mix sediment and enhance weathering

processes.

d.

Examples

(1)

root wedging

(2)

lichen / moss growth

(3)

boring clams on Oregon Coast

6.

Mineralogic Crystal Growth - mineral growth causes expansion and rock cracking

a.

Salt Wedging(1)

Precipitation of Calcium sulfate, chloride salts, and carbonates

(2)

salt precipitation from intra-fracture/inter-granular waters can have

(3)

prevalent in sandstones and porous rocks

(4)

Arid areas / coastal areas

(5)

Evidence in nature

(a)

Tafoni - weather-pitted surfaces (honey-comb) common on

sea cliffs in coastal areas

b.

Salt Hydration: hydration or addition of water to molecular structure of salt

precipitates can cause expansion

e.g. Anhydrite to Gypsum: CaSO4 + 2H2O = CaSO42H2O

c.

7.

Thermal Expansion of Salt:

(1)

salts have a relatively high coefficient of thermal expansion compared

to common rock forming silicate minerals.

(a)

i.e. salt expands more easily when heated compared to rock

material

(2)

Differential rate of thermal expansion creates stress and avenue for

physical weathering.

Clay Expansion: Wetting and Drying of Clay Minerals

23

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